High-frequency switches fabricated with microelectromechanical-systems (MEMS) technology can provide numerous benefits compared to conventional semiconductor switches. These include better amplitude-with-frequency characteristics, lower power consumption, and higher linearity.1 These characteristics make MEMS switches suitable candidates for millimeter-wave applications, provided that adequate isolation and low insertion loss are possible at those higher frequencies. As a result, a number of studies have explored the development of switches for higher-frequency bands, including compact MEMS switches.2,3

MEMS switches are usually divided into contact-type and capacitive-type switches. Contact switches typically have lower insertion losses than capacitive switches over a broad frequency ranges. However, their isolation tends to degrade at higher frequencies, owing to coupling between the open signal and the contact element beyond the desired signal. To examine the possibility of using MEMS technology for a higher-frequency switch, a lateral-contact series MEMS switch was designed for applications from 30 to 40 GHz and simulated by means of commercial computer-aided-engineering (CAE) software. Simulated results indicate low insertion loss of 0.17 dB and high isolation of 36.7 dB (both at 35 GHz), with analysis indicating that insertion loss of less than 0.2 dB and isolation of more than 37.5 dB can be achieved from 30 to 40 GHz.

Normally, the isolation of contact switches is determined by the initial capacitance between the contact metal and the coplanar-waveguide (CPW) transmission line, which is the capacitance between the input and output signal lines. In addition, the coupling capacitance between the input and output ports through the substrate on which the switch is mounted cannot be ignored. To increase the isolation of an RF MEMS switch, the coupling capacitance must be sufficiently minimized. Traditionally, switch coupling capacitance could be minimized by enlarging the gap between the signal lines of the CPW line and the contact element. Unfortunately, this further increases the driving voltage.

To overcome this tradeoff, a new method was employed by scaling the CPW lines of the switch to achieve higher isolation. Figure 1 shows a schematic view of dCPW transmission lines with different dimensions. The impedances of these CPW lines were calculated with the aid of the Advanced Design System (ADS) CAE software from Agilent Technologies; simulations were performed with the High-Frequency Simulation Software (HFSS) electromagnetic (EM) simulation software from Ansoft. The dimensions and characteristic impedances of different CPW lines are shown in the table. The levels of high isolation estimated using three-dimensional (3D) EM simulation software are shown in Fig. 2.

The inverted triangular line in Fig. 2 indicates isolation for the cases where the dimensions of a switch’s CPW line is similar to those of typical RF MEMS switches, with ground/signal/ground (G/S/G) spacing usually chosen as 60/100/60 μm (section a in Tables 1 and 2). The simulated isolation was better than 28 dB from 30 to 40 GHz. When the dimension of the CPW line were scaled to 48/80/48 μm (section b in Tables 1 and 2), the isolation increases to more than 33 dB. Then the dimension of the CPW line is scaled down to 36/60/36 μm (section c in Tables 1 and 2), and the isolation increases to more than 37.5 dB. The isolation has been improved by 9.5 dB with respect to scaling down at this point. When further scaling was done (section d in Tables 1 and 2), the isolation was only be improved by 3 dB, as the effect of the scaling down reaching its limitation.

As a result, sections c and d in Tables 1 and 2 were chosen as the dimensions of the CPW line for the proposed switch. These two sections were cascaded, as shown in Fig. 1. The dimensions of the two-section CPW line were adjusted slightly to achieve a 50-Ω characteristic impedance, which were 38/60/38 μm and 32/50/32 μm, respectively. A 10-μm gap lies between the RF contact plate of the switch and the signal line.All simulations were based on parameters experimentally measured using the fabricated gold line in the same processing wafer.